In a turbine with an axial-radial diffuser the working fluid is discharged following the last row of turbine blades and flows into an annularly flared flow passage, the diffuser, formed by an inner and an outer flow guide extending from the hub or tip of the last blade row of the turbine, respectively. The diffuser extends initially in the axial direction and circumferentially over the full 360° around the turbine rotary axis and then bends radially outwards with respect to the turbine rotary axis. The diffuser outlet typically leads to an exhaust hood, where typically the outlet is positioned within the exhaust hood. The exhaust hood in turn has an outlet to discharge the working fluid. In the case of a steam turbine, the outlet leads steam into a condenser. The exhaust hood has opposite its outlet a first portion with typically a semi-circular cross-section that encompasses half of the turbine and diffuser and a second portion with rectangular cross-section that extends from the first portion to the outlet of the exhaust hood. The transition from the first portion to the second portion of the exhaust hood is formed by two so-called throats, which are opposite from each other with respect to the turbine. The outlet is frequently arranged below the level of the turbine axis, which is frequently referred to as a downward discharging exhaust hood. However, it can also be arranged at the same level, or above the level of the turbine axis. A condenser would then be arranged adjacent on either side of the turbine or above the turbine, respectively.
The steam exiting a steam turbine after the last blade row diffuses, or decelerates, in the diffuser. As the kinetic energy of the steam flow is thus decreased in the diffuser, the static pressure rises correspondingly from the last row of turbine blades to the diffuser exit. With this increase of steam pressure in the flow direction in the diffuser there is a corresponding decrease in steam pressure at the level of the last turbine blade row as the pressure at the exhaust hood outlet is given by the cooling environment (for example the condenser) applied. Consequently, the turbine work output is increased compared to that of a turbine without a diffuser. Therefore, the pressure increase within the diffuser and the turbine power output can potentially be improved by an appropriate diffuser design.
Increasing the static pressure within the diffuser can optimize the performance of a turbine. However, losses can occur due to flow separation and vortex formations within the hood, which compromise the overall performance. Such vortices may develop to different degrees in different regions of a diffuser and exhaust hood, e.g. due to support struts or depending on the orientation of the exhaust hood. For example, in a diffuser, which leads the steam into a downward discharging exhaust hood (exhaust hood outlet below the level of the turbine), the steam diffusing in the lowest portion of the diffuser passage will enter the exhaust hood with no or very little change in flow direction. However, the steam diffusing in the uppermost portion of the diffuser and essentially being directed in the radial and vertically upward direction, experiences a change in flow direction of 180° in order to flow downwards into the exhaust hood and towards the outlet at the bottom. Such large changes in direction cause vortices and losses, which adversely affect the performance of the diffuser and consequently also the power output of the turbine.
U.S. Pat. No. 5,518,366 discloses a diffuser for a turbo machine having an inner and outer flow guide each beginning at an inlet adjacent to the last blade row of the turbine and ending at an outlet within an exhaust hood. The downward discharging exhaust hood has a flow-guiding surface that has a distance from the inlet of the outer diffuser flow guide that varies over the circumference and has a minimum of less than the length of the last turbine blade at a particular location, for example at the top of the exhaust hood. The outer flow guide has an axial length from its inlet to its outlet that also varies over the circumference of the flow guide and has a minimum at the location where the minimum distance between the flow-guiding surface of the exhaust hood and the inlet of the outer flow guide occurs. The minimum distance between the flow-guiding surface of the exhaust hood and the inlet of the outer diffuser flow guide and the axial length of the outer flow guide are defined in relation to the length of the airfoil of the last turbine blade row.